Wood products have been used as utility poles, railway ties, and construction materials in a wide variety of industries. Without proper treatment, wood products deteriorate and are susceptible to weathering, insects (e.g., termites, carpenter ants, and beetles), marine borers (e.g., mollusks and crustaceans), bacteria, and fungi (e.g., stains, white rot, soft rot, and brown rot). Wood treatment is required to prevent these problems.
Borate compounds contain oxoanions of boron in a +3 oxidation state. The simplest borate ion, BO33−, and its acidic counterpart, boric acid B(OH)3, have trigonal planar structures. Other borates include trigonal BO3 or tetrahedral BO4 structural units, sharing oxygen atoms. A number of polymeric borate ions are known. They may be made by reacting B(OH)3 or B2O3 with metal oxides. Examples include: diborate B2O54− (e.g., Mg2B2O5), triborate B3O75− (e.g., CaAlB3O7), tetraborate B4O96− (e.g., sodium tetraborate Na2B4O7.10H2O), pentaborate B5O6(OH)4− (e.g., sodium pentaborate Na[B5O6(OH)4].3H2O), and octaborate (e.g., disodium octaborate tetrahydrate, Tim-Bor, Na2B8O13.4H2O).
Borates, such as octaborate, are broad spectrum insecticides commonly used in the treatment of wood. They have the advantage of being readily diffusible into the interior of wood and exhibit low mammalian toxicity. Solid rods of boric acid, for example, are driven into the base of previously installed utility poles to prolong the life of the pole. Once inserted, the boric acid diffuses into the interior of the pole to protect the base region of the pole. However, the installation of boric acid rods is costly and labor intensive because trenches must be dug around the poles, the base must be drilled, and the rods inserted. Replacement boric acid rods must then be reinserted numerous times during the lifetime of the pole. Further, borates are susceptible to leaching and may not adequately protect against soft rot fungi.
Pentachlorophenol (PCP) is a broad spectrum biocide (e.g., insecticide, bactericide, and herbicide), used for the surface treatment of wood. PCP is an effective biocide due, in part, to its ability to inhibit oxidative phosphorylation. Solutions of PCP for the surface treatment of wood are commonly formulated in a solvent classified by the American Wood Preservative Association (AWPA) as P9 Type A oil. The AWPA P9 Type A oil comprises a “hydrocarbon solvent” and an “auxiliary solvent” which, in combination, have physical characteristics, as mandated by the AWPA, that are related to viscosity, distillation characteristics, flash point, specific gravity, and the solubility of PCP. However, PCP has the disadvantage that it is not readily diffusible into the interior of wood and may fail to protect the center “heartwood.”
As such, a single and stable wood preservative composition is needed that readily diffuses into the interior of wood to protect the heartwood, while also providing adequate treatment at the surface. Methods of applying such wood preservative composition are likewise needed that are less costly and time consuming.